DiCaprio, 40, bought the designer purse for her mother, Irmelin, reported People magazine.
The actor and Hilton stayed for the entire event, catching speeches, a musical performance by hip-hop artist Mos Def, a decadent dinner spread and all of the auctions.
"Paris was great. She bid on many things, and Leo did too," a source said. "Both her and Leo were so active during the entire auction. It was really nice to see."
Sporting a suit and his signature scruff, DiCaprio "was laughing and enjoying himself with his friends," the onlooker says.